Conventionally, a semiconductor device has a semiconductor element, which is formed in a SOI substrate. The SOI substrate is prepared such that a support substrate and an active layer are bonded to each other via an embedded insulation film. In the semiconductor device, a potential of the support substrate is fixed to be a predetermined potential such as a GND potential when the device is operated. Under a condition that the potential of the support substrate is fixed to be the predetermined potential, when a high voltage is applied to a predetermined part of the active layer, an electric charge is induced to a part of the active layer, which is adjacent to the insulation film, so that an inversion layer is formed. Accordingly, a breakdown voltage is reduced. This reduction will be explained with reference to FIG. 22.
FIG. 22 shows an equivalent electric potential distribution in the semiconductor device having a lateral type PN diode with respect to a SOI substrate J1. When a high voltage is applied to a cathode electrode J2 of the PN diode, and an anode electrode J3 is grounded, a positive charge is induced to a part of the active layer J4 adjacent to the embedded insulation film J5 so that a inversion layer is formed in the part of the active layer j4. Thus, a distance of adjacent equivalent electric potential lines between the N+ cathode region 36 and the insulation film J5 is narrowed. Accordingly, the electric field becomes high between the N+ cathode region J6 and the insulation film J5. Thus, the breakdown voltage of the device is reduced.
To prevent the breakdown voltage reduction, a semiconductor device having an insulation film, a surface of which is concaved and convexed, is disclosed in JP-B2-3959125 corresponding to U.S. Pat. No. 6,049,109. FIG. 23 shows a cross sectional view of the device. A concavity J5a and a convexity J5b are formed on the embedded insulation film J5 so that a positive charge is localized at the concavity J5a. Thus, a pseudo field plate is formed. When the pseudo field plate is formed, the equivalent electric potential lines are arranged in a vertical direction toward the convexity J5b. Accordingly, the distance between adjacent equivalent electric potential lines is modified, so that the breakdown voltage is improved.
However, when the concavity and convexity are formed on the embedded insulation film, various steps for forming the concavity and convexity are necessary. Thus, a manufacturing process is complicated. Specifically, before a silicon substrate as the active layer is bonded to the support substrate, a concavity is formed on the backside of the silicon substrate by a photo etching method. Thus, the concavity and convexity are formed on the silicon substrate in a concavity and convexity forming step. Then, in an insulation forming step, an insulation film is deposited on the backside of the silicon substrate, on which the concavity and convexity are formed. In a flattening step, the surface of the insulation film is flattened. Further, to localize the charge at the concavity, it is necessary to form the concavity having a sufficient depth in the concavity and convexity forming step. In the insulation forming step, the insulation film is formed to have a sufficient thickness so that the concavity is embedded with the insulation film. Furthermore, in the flattening step, the insulation film is flattened. Thus, the manufacturing process of the device is complicated.
Further, conventionally, a semiconductor device having a high breakdown voltage is prepared from a SOI substrate, which includes a support substrate, an active layer and an embedded insulation film. In this device, a breakdown voltage reduction at a periphery of the device may arise. In JP-B2-4204895 corresponding to US 2004/0227188, voltage-dividing diodes are arranged on a side of the high breakdown voltage device via an insulation film, the device having a rectangular shape. The diodes are coupled with each other via a wiring. In this case, the voltage-dividing diode provides to divide the voltage in multiple steps between an end of the device on the high voltage side to the other end of the device on the low voltage side in accordance with a distance. Thus, a electric potential control is performed through the side of the device, and therefore, the electric field is reduced. The breakdown voltage reduction is restricted.
However, in the above device, to control the electric potential of the side of the device, the voltage-dividing diode as a control device is additionally formed in the device. Thus, dimensions of the device increase.